P-contact with more uniform injection and lower optical loss

ABSTRACT

The current distribution across the p-layer ( 130 ) of a semiconductor device is modified by purposely inhibiting current flow through the p-layer ( 130 ) in regions ( 310 ) adjacent to the guardsheet ( 150 ), without reducing the optical reflectivity of any part of the device. This current flow may be inhibited by increasing the resistance of the p-layer that is coupled to the p-contact ( 140 ) along the edges and in the corners of contact area. In an example embodiment, the high-resistance region ( 130 ) is produced by a shallow dose of hydrogen-ion (H+) implant after the p-contact ( 140 ) is created. Similarly, a resistive coating may be applied in select regions between the p-contact and the p-layer.

FIELD OF THE INVENTION

This invention relates to the field of semiconductor light emittingdevices, and in particular to techniques for improving extractionefficiency and providing a more uniform current distribution across thelight emitting region of the device.

BACKGROUND OF THE INVENTION

The substantial increase in demand for semiconductor light emittingdevices, and the corresponding increase in competition to satisfy demandhas caused manufacturers to seek techniques that will reduce costs orimprove performance. Of particular note, techniques that improve theefficiency or quality of the emitted light may serve to distinguish onecompetitor's product from the others.

FIG. 1 illustrates an example prior art Thin Film Flip Chip (TFFC) InGaNLight Emitting Device (LED), such as disclosed in U.S. Pat. No.6,828,596, “CONTACTING SCHEME FOR LARGE AND SMALL AREA SEMICONDUCTORLIGHT EMITTING FLIp-CHIP DEVICES”, issued to Daniel A. Steigerwald,Jerome C. Bhat, and Michael J. Ludowise, and incorporated by referenceherein.

In this example device, a light emitting layer 120 is formed between ann-layer 110 and a p-layer 130. An external power source (notillustrated) provides power to the device via connections to pads 160and 170. The p-pad 160 is coupled to the p-layer 130 via a p-contact140, through an optional guard layer 150 that inhibits migration of thep-contact material. The n-contact layer 170 is coupled directly to then-layer 110 in this example. A boundary layer 180 isolates the n-contactlayer 170 and n-layer 110 from the p-layer 130 and p-contact 140.

The p-contact 140 is provided over a large area to facilitate a uniformdistribution of current through the p-layer 130, which has a relativelyhigher resistance to current flow. The n-layer 110 does not exhibit ahigh resistance, and thus the n-contact covers a smaller area, which maybe 10% or less of the device area. The p-contact 140 is preferablyhighly reflective to reflect the light toward the top, emitting surfaceof the light emitting device. Silver is commonly used as the p-contact140. The n-contact layer is also reflective and metals such as Aluminumare preferred. The guard layer 150 may be metallic, but is onlypartially reflective as a suitable highly reflective metal has not yetbeen found for this application. This partially reflective guard sheetfills the area adjacent to the p-contact, resulting in higher opticalloss at the p-contact periphery.

The inventors have recognized that the light generated within about 15microns of the periphery of the p-contact may, with high probability,enter the guard layer area 150 and suffer optical absorption beforehaving a chance to exit the device. Therefore, current injected at theedge of the p-contact will exhibit a lower external quantum efficiencythan current injected at the center area of the p-contact.

Despite the greater optical loss of the edges and corners of the device,the inventors have also noticed that more emitted light is produced atthe periphery and in the corners than at the center of the device,because the voltage drop associated with the lateral flow of currentthrough the n-contact layer, combined with the exponential dependence ofvertical current flow upon junction voltage, provides a significantlyhigher current density at the edges and in the corners of the device.These relatively high injection currents create a slight halo-effect,with bright areas in the corners of the device.

In addition to potentially introducing optical anomolies, such anon-uniform current injection pattern is inefficient, as the internalquantum efficiency is lower for higher current densities. The‘over-emitting’ portions, particularly the corners, of the lightemitting device will also be ‘hot-spots’ that draw more current in thedevice, which have been observed to lead to premature failure of devicesoperated at high current.

SUMMARY OF THE INVENTION

It would be advantageous to distance the light emission regions awayfrom the partially reflective guard layer and to further improve theuniformity of the injected current density light emissions across thesurface of the active layer.

To better address these concerns and others, in an embodiment of thisinvention, the current distribution across the p-layer of asemiconductor device is modified by purposely inhibiting current flowthrough the p-layer in regions adjacent to the guardsheet, withoutreducing the optical reflectivity of any part of the device. Thiscurrent flow may be inhibited by increasing the resistance of thep-layer that is coupled to the p-contact along the edges and in thecorners of contact area. In an example embodiment, the high-resistanceregion is produced by a shallow dose of hydrogen-ion (H+) implant afterthe p-contact is created. Similarly, a resistive coating may be appliedin select regions between the p-contact and the p-layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example,with reference to the accompanying drawings wherein:

FIG. 1 illustrates an example prior-art light emitting device.

FIG. 2 illustrates current distribution in the example light emittingdevice.

FIGS. 3A-3B illustrate an example light emitting device with a p-contactthat includes a high resistance region and a low resistance region toimprove current distribution.

FIG. 3C illustrates an alternative to FIG. 3A.

Throughout the drawings, the same reference numerals indicate similar orcorresponding features or functions. The drawings are included forillustrative purposes and are not intended to limit the scope of theinvention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation rather thanlimitation, specific details are set forth such as the particulararchitecture, interfaces, techniques, etc., in order to provide athorough understanding of the concepts of the invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced in other embodiments, which depart from these specificdetails. In like manner, the text of this description is directed to theexample embodiments as illustrated in the Figures, and is not intendedto limit the claimed invention beyond the limits expressly included inthe claims. For purposes of simplicity and clarity, detaileddescriptions of well-known devices, circuits, and methods are omitted soas not to obscure the description of the present invention withunnecessary detail.

This invention is presented in the context of the example prior artdevice of FIG. 1, for ease of illustration and understanding. One ofskill in the art will recognize, however, that some or all of theprinciples of this invention may be applicable to a variety of differentLED structures, or any structures that would benefit from a reduction inoptical loss created by an absorbing region adjacent to a low losscurrent injection region.

As noted above, the light emitting device of FIG. 1, the structure ofwhich is repeated in FIGS. 2 and 3, includes a highly reflective, largearea p-contact 140 that provides for a more uniform distribution ofcurrent through the p-layer 130. The contact between the n-layer 110 andthe n-pad 170 is along the perimeter of the n-layer 110. A boundarylayer 180 separates the n-type elements 110, 180 from the p-typeelements 130, 140, 150.

As illustrated in FIG. 2, when connected to an external source via then-pad 170 and p-pad 160, the electron current 200 from the n-pad 170spreads laterally through the n-layer 110, crossing the boundary layer180 and continuing down toward the p-contact 140 and the p-pad 160.Because the current distribution across the n-layer 110 is not perfectlyuniform, and because distance from the perimeter of the p-contact 140and the source of the current 200 is shorter than the distance from thecenter of the p-contact 140, the current flow 200 a to the perimeter ofthe p-contact 140 will be greater than the current flow 200 b to thecenter of the p-contact 140. Depending upon geometry (corner vs. edge),n-GaN sheet resistance (thickness and doping), and operating conditions(current, temperature), a substantial fraction 200 a of the currentinjection 200 may be concentrated near the boundary of the p-contact140. Accordingly, the current injection through the p-n junction ofactive layer 120 will be larger around the periphery of the active layer120, creating a higher emission of light at the periphery.

In addition to potentially objectionable optical effects caused by thisnon-uniform light emission, this non-uniformity potentially reduces theoverall light extraction efficiency, because the higher light emissionoccurs in regions where the optical losses are greatest. At the centerof the light emitting active layer 120, most of the emitted light willeventually exit the top surface of the light emitting device, eitherdirectly, or via reflections from the p-contact layer 140. Light that isemitted from the center of the active layer 120 at severe angles(side-light) relative to the top surface will have a greater likelihoodof exiting the top surface of the device than such light from otherregions, because, from the center, there is less likelihood ofencountering a light absorbing feature, such as the boundary layer 180,before exiting the top surface. Conversely, along the perimeter of theactive layer 120, the likelihood of encountering the boundary layer 180is significantly higher, with a corresponding increase in optical loss.

In addition to the optical problems associated with the non-uniformcurrent flow, the larger current flow 200 a creates a “hot spot” thatlowers the bandgap and draws even more current, resulting in thecreation of failure prone areas in the device.

Additionally, the uneven current injection into the light emittingregion also reduces the overall chip internal quantum efficiency (IQE; aratio of the number of photons emitted per injected electron), becausethe IQE decreases as the current density increases (known in that art as“IQE droop”).

In an embodiment of this invention, hole current injection is inhibitedin the periphery region 310 of the p-contact 140, as illustrated inFIGS. 3A-3B, FIG. 3B being a cross section A-A′ of the device of FIG.3A. This hole current injection inhibition region 310 may be formed byusing, for example, a shallow low dose H+ implant, or other means ofreducing, or blocking, current flow in this region. Such an implant maybe performed after a silver deposition to form the p-contact 140, usinga photo-resist pattern to form the region 310 that is subsequentlyprocessed to create the current-inhibiting region 310. Sufficient energyand dose for this purpose depends upon the Ag thickness but a 15 keVenergy and a dose of 2e14 cm−2 are nominal values. High energy thatimplants deeper than 50 nm into the p-layer and high doses will createexcessive damage in the p-layer and increase optical absorption.

Other means of inhibiting current flow to the p-layer 130 at theperiphery may also be used, such as coating the periphery of thep-contact 140 with a resistive material 310′, such as a dielectric orother poorly conductive transparent material, as illustrated in FIG. 3C.The p-contact layer 140 may run up over the edge of the dielectric layer310′ overlapping 310′ to an extent of at least 5 μm, creating in theoverlapped areas a highly reflective Ag-dielectric mirror.

By inhibiting the current flow in the region 310, the source current 300is forced to be laterally diverted further through the n-layer 110, asillustrated by the current flows 300 a, 300 b in FIG. 3A. Because of thelateral diversion from the periphery of the p-contact 140, the current300 a flows farther through the n-layer 110 before reaching thep-contact 140 than the current 200 a in FIG. 2, and will correspondinglybe reduced in magnitude. This reduction in current magnitude at theperiphery will reduce the ‘hot-spot’ associated with the high current200 a, and will reduce the likelihood of premature failure caused by thehigh current 200 a.

The reduction in current at the periphery of the p-contact 140 willcorrespondingly provide an increase in the current 300 b that flows tothe center of the light emitting layer 120, compared to the current 200b in FIG. 2. The overall effect, for the same amount of total current inFIGS. 2 and 3, is a more uniform excitation of the light emitting layer120 of FIG. 3, which provides for a more uniform light output from thedevice of FIG. 3.

Additionally, by laterally shifting the current away from the peripheryof the p-contact 140, the edge of the light emission region is relocatedaway from the absorbing guard region 150, thereby reducing the amount oflight that is lost to this region 150.

It is desirable to maintain as small a radius of curvature as possibleat the outer corners 320 of the p-contact layer, so as to provide amaximal reflective area below the light emitting layer 120, therebyminimizing losses for any backscattered light. However, in aconventional device, a small radius of curvature maximizes the currentcrowding in the corners 320 of the device, causing even greater localhotspots at the corners. A reduction in the likelihood of localhot-spots may also be achieved by rounding the inner corners 330 of theinhibition region 310. By creating a current inhibiting region of largerradius of curvature at the corner 330 upon a p-contact layer with asmall radius of curvature at the corners 320, the optical efficiency ismaintained, and hot spots are mitigated.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

For example, it is possible to operate the invention by situating acontact enhancing layer, such as NiO, beneath the regions of the Agcontact where an enhanced contact is desired and eliminating this layerin the regions where the enhancement is not desired. This embodiment maybe combined with a reduction in Mg doping or other impairment in thetypical p-contact to reduce the effectiveness of the Ag—GaN contact.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measured cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope.

1. A light emitting device comprising: an n-layer, a p-layer, a lightemitting layer between the n-layer and the p-layer, an n-pad forcoupling to the n-layer, a p-pad for coupling to the p-layer, and ap-contact that couples the p-pad to the p-layer to facilitate currentinjection through the p-layer, wherein the p-contact is configured toinhibit current injection through the player in at least onecurrent-inhibiting region of the p-contact, and the at least onecurrent-inhibiting region includes an ion-injected region of thep-contact.
 2. The light emitting device of claim 1, wherein thecurrent-inhibiting region corresponds to a region of the p-contact thatprovides maximum current injection in the absence of thecurrent-inhibiting regions.
 3. The light emitting device of claim 1,wherein the current-inhibiting region corresponds to a periphery of thep-contact.
 4. (canceled)
 5. The light emitting device of claim 1,wherein the p-contact includes silver.
 6. A light emitting devicecomprising: an n-layer, a p-layer, a light emitting layer between then-layer and the p-layer, an n-pad for coupling to the n-layer, a p-padfor coupling to the p-layer, and a p-contact that couples the p-pad tothe p-layer to facilitate current injection through the p-layer, whereinthe p-contact is configured to inhibit current injection through thep-layer in at least one current-inhibiting region of the p-contact, andwherein the p-contact is reflective of light emitted by the lightemitting layer and the current-inhibiting region includes asubstantially transparent resistive coating between the p-contact andthe p-layer.
 7. (canceled)
 8. The light emitting device of claim 1,wherein the current-inhibiting region includes curved corners with aradius of curvature greater than a radius of curvature of the p-contact.9. The light emitting device of claim 1, wherein the current-inhibitingregion corresponds to a region of the p-contact closest to a contactregion between the n-pad and the n-layer.
 10. A light emitting devicecomprising: an n-layer, a p-layer, a light emitting layer between then-layer and the p-layer, an n-pad for coupling to the n-layer, a p-padfor coupling to the p-layer, and a p-contact that couples the p-pad tothe p-layer to facilitate current injection through the p-layer, whereinthe p-contact is configured to inhibit current injection through thep-layer in at least one current-inhibiting region of the p-contact andthe p-contact includes a material that improves ohmic contact with thep-layer, and the current-inhibiting region corresponds to an absence ofthis material.
 11. The light emitting device of claim 10, wherein thematerial that improves ohmic contact includes NiO.
 12. A methodcomprising: creating a light emitting element comprising an activeregion between an n-layer and a p-layer, providing a p-contact to thep-layer, the p-contact being coupled to the p-layer so as to provide anon-uniform current flow from the p-contact to the p-layer, providing ap-pad coupled to the p-contact to facilitate coupling to an externalsource of power, and providing an n-pad coupled to the n-layer tofacilitate coupling to the external source of power, wherein thenon-uniform current flow is provided by creating at least onecurrent-inhibiting region of the p-contact, and the method includesimplanting ions in the p-layer to create the current-inhibiting region.13. (canceled)
 14. (canceled)
 15. The method of claim 14, wherein thep-contact includes silver.
 16. The method of claim 12, wherein thecurrent-inhibiting region corresponds to a periphery of the p-contact.17. The method of claim 12, wherein the current-inhibiting regionincludes curved corners with a radius of curvature greater than a radiusof curvature of the p-contact.
 18. The method of claim 12, wherein thecurrent-inhibiting region corresponds to a region of the p-contactclosest to a contact region between the n-pad and the n-layer.
 19. Amethod comprising: creating a light emitting element comprising anactive region between an n-layer and a p-layer, providing a p-contact tothe p-layer, the p-contact being coupled to the p-layer so as to providea non-uniform current flow from the p-contact to the p-layer, providinga p-pad coupled to the p-contact to facilitate coupling to an externalsource of power, and providing an n-pad coupled to the n-layer tofacilitate coupling to the external source of power, wherein thenon-uniform current flow is provided by creating at least onecurrent-inhibiting region of the p-contact, the method includingproviding a material that improves ohmic contact between the p-contactand p-layer, and omitting that material to form the current-inhibitingregion.
 20. The method of claim 19, wherein the material that improvesohmic contact includes NiO.
 21. The light emitting device of claim 1,wherein the current-inhibiting region of the p-contact serves to improveuniformity of current injection through the light emitting layer. 22.The light emitting device of claim 6, wherein the current-inhibitingregion of the p-contact serves to improve uniformity of currentinjection through the light emitting layer.
 23. The light emittingdevice of claim 10, wherein the current-inhibiting region of thep-contact serves to improve uniformity of current injection through thelight emitting layer.